Acetyl CoA carboxylase (ACCase) is an enzyme involved in many important metabolic pathways in plant, animal and bacterial cells. The enzyme is especially important in fatty acid synthesis in plants and is sensitive to inhibition by some types of herbicides. Structurally, ACCases are biotinylated and are quite large enzymes consisting of one or more subunits. For example, most ACCases of animals, higher plants, and yeast are dimers of 420 to 700 kD native MW and contain subunits of 200 to 280 kD. Diatom and algal ACCases are 700 to 740 kD tetramers of 160 to 180 kD subunits. Bacterial ACCase consists of three dissociable proteins; biotin carboxylase (51 kD), biotin carboxyl carrier protein (22.5 kD), and biotin transcarboxylase (130 kD).
Acetyl CoA Carboxylase (ACCase) catalyzes the formation of malonyl-CoA from acetyl-CoA and bicarbonate in animal, plant, and bacterial cells. Malonyl-CoA is an essential substrate for (i) de novo fatty acid (FA) synthesis, (ii) fatty acid elongation, (iii) synthesis of secondary metabolites such as flavonoids and anthocyanins, and (iv) malonylation of some amino acids and secondary metabolites. Synthesis of malonyl-CoA is the first committed step of flavonoid and fatty acid synthesis and current evidence suggests that ACCase catalyzes the rate-limiting step of fatty acid synthesis. Formation of malonyl-CoA by ACCase occurs via two partial reactions and requires a biotin prosthetic group:
(i) E-biotin+ATP+HCO.sub.3 .fwdarw.E-biotin-CO.sub.2 +ADP+Pi PA1 (ii) E-biotin-CO.sub.2 +Acetyl-CoA.fwdarw.E-biotin+malonyl-CoA PA1 (NET) Acetyl-CoA+ATP+HCO.sub.3 .fwdarw.malonyl-CoA+ADP+Pi PA1 (1) an increase or decrease in expression of an altered plant acetyl CoA carboxylase gene; or PA1 (2) by introducing an altered or functional mutant plant acetyl CoA carboxylase gene.
In E. coli, these reactions are catalyzed by three distinct components; biotin carboxylase, biotin transcarboxylase, and biotin carboxyl carrier protein, which can be separated and yet retain partial activity. Plant and animal ACCases contain all three activities on a single polypeptide.
In plants, most ACCase activity is located in plastids of green and non-green plant tissues including leaves and oil seeds. Leaf ACCase activity is primarily located in mesophyll cells, but lesser amounts have been found in C-4 bundle sheath cells and in epidermal cells. The subcellular location of ACCase activity in epidermal cells is unknown, but since synthesis of very long-chain fatty acids (VLCFA) for formation of waxes, cutin, and suberin occurs on the endoplasmic reticulum (ER), malonyl-CoA might also be derived from a cytosolic ACCase. In contrast, rat ACCase is primarily cytosolic or associated with the outer mitochondrial membrane.
De novo fatty acid synthesis in chloroplasts involves successive 2-carbon additions to acetate, using malonate as the 2-C donor. All intermediates are attached to acyl carrier protein (ACP). Synthesis in plastids resembles that in E. coli in that the fatty acid synthesis complex can be dissociated into separate enzymes: .beta.-ketoacyl-ACP synthase (KAS), .beta.-ketoacyl-ACP reductase, .beta.-hydroxyl-ACP dehydratase, and enoyl-ACP reductase, acetyl-CoA:ACP transacylase, and malonyl-CoA:ACP transacylase. A highly active KASIII isozyme catalyzes the condensation of acetyl-CoA and malonyl-ACP. Successive additions of malonyl-CoA to acyl-ACPs catalyzed by KAS I form C16 acyl-ACP, some of which is converted to C18 acyl-ACP by KAS II and then to C18:1-ACP. Fatty acid metabolism then diverges; de-esterification allows movement to the cytoplasm (eukaryotic path) where fatty acids may be further unsaturated and/or elongated by additions of malonyl-CoA in the ER. Alternatively, fatty acids are linked to glycerol-3-phosphate (prokaryotic path), further unsaturated, and used for synthesis of chloroplast lipids. A portion of cytoplasmic lipids returns to the chloroplast. The relative contributions of these two paths are species-specific but appear to be relatively flexible in mutants blocked in either path. In oil-storing organs such as cotyledons and monocot embryos, the triacylglycerides are stored in cytoplasmic oil bodies surrounded by a single unit membrane.
Condensation of malonyl-CoA with phenylpropionyl-CoAs or acetyl-CoA leads to synthesis of flavonoids, anthocyanins, or to polyacetates. Condensation is increased by light, elicitors, or pathogens and may be the rate-limiting step in synthesis of some phytoalexins. In addition to the secondary metabolites derived by de novo synthesis, malonyl conjugates of flavonoid glycosides, formed by malonyl-CoA:flavonoid glycoside malonyltransferase, D-amino acids and 1-amino-carboxyl-cyclopropane (ethylene precursor) are found in plants. Malonylated compounds accumulate in vacuoles, probably after synthesis in the cytoplasm.
An important property of ACCase is the central role it plays in fatty acid synthesis and accumulation in plants and seeds. Available evidence supports the idea that ACCase activity is the rate-limiting step for de novo fatty acid synthesis in plants. High rates of ACCase activity in vitro parallel or slightly precede high rates of lipid deposition or [.sup.14 C]acetate incorporation into lipids in developing leaves and oil seeds. Significant changes in plant ACCase activity occur during chloroplast development and increase in ACCase activity correlates with lipid deposition in developing oil seeds. Turnham et al., Biochem. J., 212:223 (1883); and Beittenmiller et al., Plant Physiol., 100:923 (1992).
Among other properties, ACCase in most monocots is also inhibited by several herbicides. [.sup.14 C]acetate incorporation into maize lipids is strongly inhibited by fluazifop and sethoxydim due to inhibition of plastid ACCase. In barley, however, fluazifop had little effect on [.sup.14 C]acetate incorporation into very long-chain fatty acids. Since synthesis of very long-chain fatty acids occurs in the cytosol on the ER, and de novo fatty acid synthesis occurs in the plastids, cytosolic malonyl-CoA might be supplied by a herbicide insensitive ACCase isozyme.
There are three general mechanisms by which plants may be resistant to, or tolerant of, herbicides. These mechanisms include insensitivity at the site of action of the herbicide (usually an enzyme), rapid metabolism (conjugation or degradation) of the herbicide, or poor uptake and translocation of the herbicide. Altering the herbicide site of action from a sensitive to an insensitive form is the preferred method of conferring tolerance on a sensitive plant species. This is because tolerance of this nature is likely to be a dominant trait encoded by a single gene, and is likely to encompass whole families of compounds that share a single site of action, not just individual chemicals. Therefore, detailed information concerning the biochemical site and mechanism of herbicide action is of great importance and can be applied in two ways. First, the information can be used to develop cell selection strategies for the efficient identification and isolation of appropriate herbicide-tolerant variants. Second, it can be used to characterize the variant cell lines and regenerated plants that result from the selections.
Tissue culture methods have been used to select for resistance (or tolerance) using a variety of herbicides and plant species (see review by Meredith and Carlson, 1982, in Herbicide Resistance in Plants, eds. Lebaron and Gressel, pp. 275-291, John Wiley and Sons, NY). For example, P. C. Anderson et al., in U.S. Pat. No. 4,761,373, disclose the use of tissue culture methods to produce maize plants resistant to herbicidal imidazolidones and sulfonamides. The resistance is due to the presence of altered acetohydroxy acid synthase which is resistant to deactivation by these herbicides.
Certain 1,3-cyclohexanediones exhibit general and selective herbicidal activity against plants. One such cyclohexanedione is sethoxydim {2-[1-(ethoxyimino)butyl]-5-[2-(ethylthio)propyl]-3-hydroxy-2-cyclohexen-1 -one}. Sethoxydim is commercially available from BASF (Parsippany, N.J.) under the designation POAST.TM..
Other herbicidal cyclohexanediones include clethodim, (E,E)-(.+-.)-2-[1-[[(3-chloro-2-propenyl)oxy]imino]propyl]-5-[2-(ethylthio )propyl]-3-hydroxy-2-cyclohexen-1-one; available as SELECT.TM. from Chevron Chemical (Valent) (Fresno, Calif.); cloproxydim, (E,E)-2-[1-[[(3-chloro-2-propenyl)oxy]imino]butyl]-5-[2-(ethylthio)propyl] -3-hydroxy-2-cyclohexen-1-one; available as SELECTONE.TM. from Chevron Chemical (Valent) (Fresno, Calif.); and tralkoxydim, 2-[1-(ethoxyimino)propyl]-3-hydroxy-5-mesitylcyclohex-2-enone, available as GRASP.TM. from Dow Chemical USA (Midland, Mich.).
For purposes of reference in the present specification, the herbicides described in the two preceding paragraphs and other structurally related herbicidal compounds, are collectively referred to as the cyclohexanedione family of herbicides.
Certain aryloxyphenoxypropanoic acids exhibit general and selective herbicidal activity against plants. In these compounds, the aryloxy group may be phenoxy, pyridinyloxy or quinoxalinyl. One such herbicidal aryloxyphenoxypropanoic acid is haloxyfop, {2-[4-[[3-chloro-5-(trifluoromethyl)-2-pyridinyl]oxy]phenoxy]propanoic acid}, which is available as VERDICT.TM. from Dow Chemical USA (Midland, Mich.). Another is diclofop, {(.+-.)-2-[4-(2,4-dichlorophenoxy)-phenoxy]propanoic acid}, available as HOELON.TM. from Hoechst-Roussel Agri-Vet Company (Somerville, N.J.).
Other members of this family of herbicides include fenoxyaprop, (.+-.)-2-[4-[(6-chloro-2-benzoxazolyl)oxy]phenoxy]propanoic acid; available as WHIP.TM. from Hoechst-Roussel Agri-Vet Company (Somerville, N.J.); fluazifop, (.+-.)-2-[4-[[5-(trifluoromethyl)-2-pyridinyl]oxy]phenoxy]propanoic acid; available as FUSILADE.TM. from ICI Americas (Wilmington, Del.); fluazifop-P, (R)-2-[4-[[5-(trifluoromethyl)-2-pyridinyl]oxy]phenoxy]propanoic acid; available as FUSILADE 2000.TM. from ICI Americas (Wilmington, Del.); and quizalofop, (.+-.)-2-[4[(6-chloro-2-quinoxalinyl)oxy]phenoxy]propanoic acid; available as ASSURE.TM. from E. I. DuPont de Nemours (Wilmington, Del.).
For purposes of reference in the present specification, the herbicides referred to in the two preceding paragraphs and other structurally related herbicidal compounds, are collectively referred to as herbicidal aryloxyphenoxypropanoic acids.
Thus, there is a need for methods to develop plants that are resistant or tolerant to herbicides. There is also a need to increase the oil and/or fatty acid content of the plants and seeds, as well as for methods to increase their resistance to herbicides. There is a need to identify and clone genes important in conferring herbicide tolerance and in increasing the oil content of plants.